The exposure apparatus for production of semiconductor devices is arranged to project and transcribe a circuit pattern formed on a mask, onto a photosensitive substrate such as a wafer coated with a resist, through a projection optical system.
With the exposure apparatus of this type, there are desires for a further improvement in resolving power as the circuit pattern to be transcribed becomes finer and finer, and light of a shorter wavelength is being used as exposure light. In recent years, the exposure apparatus using the EUV (Extreme UltraViolet) light with the wavelength of approximately 5 to 50 nm (hereinafter referred to as “EUVL (Extreme UltraViolet Lithography) exposure apparatus”) has been proposed as next-generation equipment.
At present, there are the following three types of light sources proposed heretofore, as light sources for supplying the EUV light:
(1) Light sources for supplying SR (synchrotron radiation);
(2) LPP (Laser Produced Plasma) light sources for obtaining the EUV light by focusing a laser beam on a target to turn the target into a plasma;
(3) DPP (Discharge Produced Plasma) light sources for obtaining the EUV light by applying a voltage on an electrode of a target substance, or between electrodes in a state in which a target substance is present between the electrodes, to turn the target material into a plasma.
The DPP light sources and LPP light sources will be generically referred to hereinafter as “plasma light sources.”
The EUV light is isotropically radiated from a plasma light source. Namely, the plasma light source can be regarded as a point light source. The size (diameter) of the plasma light source is approximately from 50 to 500 μm.
FIG. 1 shows an example of a conventional collector optical system. A collective mirror 2 has a reflecting surface shaped in an ellipsoid of revolution, and, when the plasma light source 1 is positioned at the first focal point of the ellipsoid (hereinafter referred to as the first focal point), the EUV light reflected on the collective mirror 2 is collected at the second focal point of the ellipsoid (hereinafter referred to as the second focal point) to form a light source image 3 there. A stop 7 for blocking beams directly incident from the EUV light source 1 without being collected is located on a plane passing the second focal point of the collective mirror 2 and being perpendicular to the optical axis (hereinafter referred to as a second focal plane). An illumination optical system is located downstream of this stop 7.
In the above-described collector optical system as shown in FIG. 1, most of beams traveling from plasma light source 1 to the downstream area (to the right in FIG. 1) are not collected by the collective mirror 2 to be wasted. A conceivable means for collecting such beams traveling to the downstream area as well is the collector optical system as shown in FIG. 2.
Part of the EUV light diverging isotropically is also collected by collective mirror 2 having a reflecting surface shaped in an ellipsoid of revolution similar to the conventional collective mirror shown in FIG. 1, to form a light source image 3 on the second focal plane. On the other hand, other part of the EUV light is reflected by an auxiliary collective mirror 4 having a reflecting surface of spherical shape centered on the position of plasma light source 1, to be once collected at the same position as the plasma light source 1 is located. Thereafter, the light is reflected by the collective mirror 2 to be focused at the same position as the light source image 3 is formed.
Namely, a real image of the plasma light source 1 by the collective mirror 2 and a real image by an optical system as a combination of auxiliary collective mirror 4 with collective mirror 2 are formed as superimposed at the position of light source image 3.
Since this collector optical system is able to guide the beams diverging from the plasma light source 1, over a wider range of solid angles to the light source image 3, the quantity of light guided to the illumination optical system increases. The collector optical systems based on this idea are already put in practical use as collector optical systems of projectors or the like (e.g., production information of Panasonic DLP projector Lightia TH-D9610J [browsed May 26, 2003], Internet <http://panasonic.biz/projector/lightia/d9610/kido.html>).
The plasma used as an EUV light source strongly absorbs the EUV light in general. The reason why the EUV light of a specific wavelength is generated from the plasma is that an electron transits between energy levels inherent to an atom. Light is generated upon a transition to a lower energy level, whereas light of the same wavelength is absorbed upon a transition to a higher energy level. Therefore, light generated from a plasma is essentially largely absorbed by the plasma.
Therefore, the collector optical system of FIG. 2 cannot be applied directly to the EUV exposure apparatus.
The reason is that in the collector optical system of FIG. 2 the EUV light reflected by auxiliary collective mirror 4 and returning to the position of EUV light source 1 is absorbed by the plasma and thus cannot reach the position of the light source image 3 in fact. Accordingly, there is no increase in the total quantity of the collected EUV light.